26,001 reputation
162134
bio website lightandmatter.com
location Fullerton, California
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visits member for 3 years, 4 months
seen Nov 25 at 1:48

I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.


Nov
17
comment How, in practice, could instantaneous signalling violate causality?
It would help if you defined what frame the signaling was going to be instantaneous in. The frame of the transmitter? Of the receiver?
Nov
17
comment Why the electric field $\vec{E}$ is constant (=position independent) for an infinite 2D sheet of constant charge?
You can get this result without Gauss's law, just on dimensional grounds. There's no way to put together any equation in terms of the charge density and the distance $r$ from the sheet that will have the right units and vary with $r$.
Nov
17
comment Twin paradox where the twins start at different locations
The phrases "in the same reference frame" and "enters A's reference frame" seem to show a misconception about what a frame of reference is. Any physical sequence of events can be described in any frame of reference. Changing frames of reference isn't a change of physical state.
Nov
17
comment How are the Weyl & Riemann curvature tensors related to the stress energy tensor in GR?
@brightmagus: In addition to what Rod said, it's possible to have primordial gravitational waves left over from the big bang. In a maximum-entropy big bang, almost all the energy would have been in this form.
Nov
17
revised Can black holes form in a finite amount of time?
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Nov
17
revised Can black holes form in a finite amount of time?
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Nov
16
revised Can black holes form in a finite amount of time?
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Nov
16
comment How can anything ever fall into a black hole as seen from an outside observer?
This answer is along the lines of "my friend the cosmologist said..." or "I think what my friend said was..." Not very convincing. We have better answers to the question that this one duplicates: physics.stackexchange.com/questions/5031/…
Nov
16
comment How can anything ever fall into a black hole as seen from an outside observer?
This answer is completely wrong. There are several correct answers by people competent in relativity for the question that this one duplicates: physics.stackexchange.com/questions/5031/…
Nov
16
comment Can black holes form in a finite amount of time?
duplicated by physics.stackexchange.com/q/21319
Nov
16
revised Can black holes form in a finite amount of time?
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Nov
16
comment Can black holes form in a finite amount of time?
@Anixx: Exactly the opposite is the case. Black hole evaporation is the only thing that ever makes it possible for a distant observer to say yes, the infalling matter has definitely hit the singularity. See figure 3 in my answer and the explanation below.
Nov
16
revised Can black holes form in a finite amount of time?
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Nov
16
revised Can black holes form in a finite amount of time?
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Nov
16
comment What's the escape velocity of Naked Singularities?
For the definition of a naked singularity, see Penrose, "Gravitational Collapse," 1973, adsabs.harvard.edu/full/1974IAUS...64...82P
Nov
16
comment How do tachyons violate causality?
Does imaginary time mean time is going backward? No, a sign flip would mean that it was going backward. The fact that the equation for time dilation misbehaves for $v\ge c$ is an indication that it is not trivial to extend the Lorentz transformation to velocities $>c$. It can in fact be done in 1+1 dimensions (although not just be plugging $v>c$ into the same equations). However, there is a no-go theorem that shows it can't be done in $3+1$ dimensions: V. Gorini, "Linear Kinematical Groups," Commun Math Phys 21 (1971) 150
Nov
16
comment Rotating black holes and naked singularity
The Chappell paper isn't dated, but from the references it looks like it's from about 1998, which means it's extremely out of date. There has been a lot more work on this kind of thing recently by people like Hubeny and Poisson. Some more recent references show that the situation is far from settled: arxiv.org/abs/1211.3889 , arxiv.org/abs/1309.2027
Nov
16
comment Rotating black holes and naked singularity
possibly relevant: physicsforums.com/showthread.php?p=3445316
Nov
16
comment What's the escape velocity of Naked Singularities?
What makes you think that a naked singularity should have a well-defined escape velocity? By the simplest definition of a naked singularity, the big bang singularity actually qualifies. (Some extra work is required in order to define a naked singularity that forms by gravitational collapse from nonsingular initial conditions.) Even in Newtonian gravity, escape velocity depends on where you start -- how far from the gravitating object.
Nov
16
answered Why isn't our universe symmetric?